The present invention relates to a method and apparatus for producing high quality laser-induced damage images in transparent objects.
A number of techniques for creating a variety of patterns on the surface and inside of transparent substrates using pulsed laser radiation are well known.
One publication disclosing such techniques is the Russian invention # 321422 to Agadjanov et. al., published on Nov. 16, 1970 (#140454529-33). The invention concerns a method of manufacturing decorative products inside a transparent material by changing the material structure by laser radiation. As disclosed, by moving a material relative to a focused laser beam, it is possible to create a drawing inside the material.
U.S. Pat. No. 4,092,518 to Merard discloses a method for decorating transparent plastic articles. This technique is carried out by directing a pulsed laser beam into the body of an article by successively focusing the laser beam in different regions within the body of the article. The pulse energy and duration is selected based upon the desired extent of the resulting decorative pattern. The effect of the laser is a number of three dimensional xe2x80x9cmacro-destructionxe2x80x9d (fissures in the material of the article) appearing as fanned-out cracks. The pattern of the cracks produced in the article is controlled by changing the depth of the laser beam focus along the length of the article. Preferably, the article is in the form of a cylinder, and the cracks are shaped predominantly as saucer-like formations of different size arranged randomly around the focal point of the optical system guiding a laser beam. The device used to carry out this technique is preferably a multi-mode solid-state, free-running pulse laser used in conjunction with a convergent lens having a focal length from 100 to 200 mm.
U.S. Pat. No. 4,843,207 to Urbanek et al., discloses a method of creating controlled decorations on the surface of a hollow symmetrical transparent article. This technique is preferably carried out on glass. The glass is preconditioned with a coating on the outer surface of the glass being approximately 1.2 mm thick and made of a material having at least 75% absorption of laser radiation. The technique is also carried out using a laser having a wave of length of 0.5 to 2 microns acting upon the external coating through the wall of the cylindrical glass article. The laser beam moves so that it is focused on the surface of the cylinder, and moves about the axis of symmetry of the cylinder to irradiate the aforementioned surface coating. As a result, the irradiated portions of the surface coating go through a phase change and a pattern is formed.
U.S. Pat. No 5,206,496 to Clement et al. discloses a method and apparatus for providing in a transparent material, such as glass or plastic, a mark which is visible to the naked eye or which may be xe2x80x9cseenxe2x80x9d by optical instruments operating at an appropriate wavelength. The Clement et al. Patent describes a method and apparatus for producing a subsurface marking which is produced in a body such as bottle, by directing into the body a high energy density beam and bringing the beam to focus at a location spaced from the surface, so as to cause localized ionization of the material. In the preferred embodiment the apparatus includes a laser as the high energy density beam source. The laser may be a Nd-YAG laser that emits a pulsed beam of laser radiation with a wavelength of 1064 nm. The pulsed beam is incident upon a first mirror that directs the beam through a beam expander and a beam combiner to a second mirror. A second source of laser radiation in the form of a low power Hexe2x80x94Ne laser emits a secondary beam of visible laser radiation with a wavelength of 638 m. The secondary beam impinges upon the beam combiner where it is reflected toward the second reflecting surface coincident with the pulsed beam of laser radiation from the Nd-YAG laser. The combined coincident beams are reflected at the reflecting surface via reflecting two other surfaces to a pair of movable mirrors for controlling movement of the beam. The beam then passes through a lens assembly into the body to be marked.
Soviet patent publication 1838163 to P. V. Agrynsky, et. al discloses a process for forming an image in a solid media by processing of the optically transparent solid material by a beam of radiation with changeable energy for creation of the image.
WIPO Patent Document No. 96/30219 to Lebedev et al. discloses a technology for creating two- or three-dimensional images inside a polymer material using penetrating electromagnetic radiation. The technology can be used for marking and for producing decorative articles and souvenirs. Specifically, laser radiation is used as the penetrating radiation, and carbonizing polymers are used as the polymer material. By these means, it is possible to produce both black and half-tone images in the articles.
U.S. Pat. No. 5,575,936 to Goldfarb discloses a process and apparatus where a focused laser beam causes local destruction within a solid article, without effecting the surface thereof. The apparatus for etching an image within a solid article includes a laser focused to a focal point within the article. The position of the article with respect to the focal point is varied. Control means, coupled to the laser, and positioning means are provided for firing the laser so that a local disruption occurs within the article to form the image within the article.
U.S. Pat. No. 5,637,244 to Erokhin discloses a technique which depends on a particular optical system including a diffraction limited Q-switched laser (preferably a solid-state single-mode TEM00) aimed into a defocusing lens having a variable focal length to control the light impinging on a subsequent focusing lens that refocuses the laser beam onto the transparent article being etched. The laser power level, operation of the defocusing lens, and the movement of the transparent article being etched are all controlled by a computer. The computer operates to reproduce a pre-programmed three-dimensional image inside the transparent article being etched. In the computer memory, the image is presented as arrays of picture elements on various parallel planes. The optical system is controlled to reproduce the stored arrays of picture elements inside the transparent material. A method for forming a predetermined half-tone image is disclosed. Accordance to the method, microdestructions of a first size are created to form a first portion of the image and microdestruction of a second size different from the first size are created to form a second portion of the image. Different sizes of microdestructions are created by changing the laser beam focusing sharpness and the radiation power thereof before each shot.
U.S. Pat. No. 5,886,318 to A. Vasiliev and B. Goldfarb discloses a method for laser-assisted image formation in transparent specimens, which consists in establishing a laser beam having different angular divergence values in two mutually square planes. An angle between the plane with a maximum laser beam angular divergence and the surface of the image portion being formed is changed to suit the required contrast of an image.
EPO Patent Document 0743128 to Balickas et al. disclose a method of marking products made of transparent materials which involves concentration of a laser beam in the material which does not absorb the beam, at a predetermined location, destruction of the material by laser pulses and formation of the marking symbol by displacement of the laser beam. Destruction of the material at that location takes place in two stages. In the first stage, the resistance of the material to laser radiation is altered, while, in the second stage, destruction of the material takes place at that location.
Russian patent publication RU 20082288 to S. V. Oshemkov discloses a process for laser forming of images in solid media by the way of focusing of laser radiation in a point inside a sample which differs by following: with the aim to save the surface and the volume of the sample before the definite point and creation of three dimensional images, the sample is illuminated with the power density higher than the threshold of volume breakdown and moving the sample relatively to the laser beam in three orthogonal directions.
U.S. Pat. No. 6,087,617 to Troitski et al. discloses a computer graphic system for producing an image inside optically transparent material. An image reproducible inside optically transparent material by the system is defined by potential etch points, in which the breakdowns required to create the image in the selected optically transparent material are possible. The potential etch points are generated based on the characteristics of the selected optically transparent material. If the number of the potential etch points exceeds a predetermined number, the system carries out an optimization routine that allows the number of the generated etch points to be reduced based on their size. To prevent the distortion of the reproduced image due to the refraction of the optically transparent material, the coordinates of the generated etch points are adjusted to correct their positions along a selected laser beam direction.
U.S. patent application Ser. No. 09/354,236 to Troitski discloses a laser-computer graphic system for generating portrait and 3-D reproductions inside optically transparent material. Accordance to the invention, production of a portrait of the same resolution like a computer image is made by using a multi-layer picture. Points of every layer are arranged so that the distance between adjacent etch points are equal to the minimal distance between etch points that can be provided without the breakage of the material. Every layer is parallel with respect to the portrait plane, and distance between parallel planes is set equal to minimal distance at which the breakage of the material does not occur.
U.S. patent application Ser. No. 09/356,252 to Troitski discloses method and laser system for generating the etch points with improved characteristics inside optically transparent material. According to the invention two laser beams are directed at the same focal point inside transparent material. The first and the second laser beams have energy levels below an energy level sufficient to cause breakdown of the material but combination of their energy levels is greater than breakdown threshold. The first and the second laser beams are directed at the same point in directions generally perpendicular to one another.
U.S. patent application Ser. No. 09/557,306 to Troitski discloses method and laser system for creation of laser-induced damages to produce high quality images. Accordance to the invention, a laser-induced damage is produced by simultaneously generating breakdowns in several separate focused small points inside the transparent material area corresponding to this etch point. Damage brightness is controlled by variation of a number of separate focused small points inside the transparent material area.
U.S. patent application Ser. No. 09/583,454 to Troitski discloses method and laser system controlling breakdown process development and space structure of laser radiation for production of high quality laser-induced damage images. Accordance to the invention, at the beginning an applied laser radiation level just exceeds an energy threshold for creating a plasma condition in the material, and thereafter the energy level of the applied laser radiation is just maintain the plasma condition. Accordance to another method a laser generates a TEMmn radiation. The values of the integers m and n are controlled and determined so as to reproduce particular gray shades for a particular point of an image.
The patents referred above disclose methods and systems producing laser-induced damage images by using Nd-YAG laser operating at a wavelength of 1064 nm and generating an emission having a pulse duration of several tens of nanoseconds. These systems focus beam at a spot of relatively large volume that creates laser-induced damages of relatively large scale. Consequently an image is a block of relatively large damages and different shades of gray are reproduced by variation of damage brightness. Such task solution of creation of laser-induced damage images (the selection of a wavelength, a pulse duration and a focal volume) results from numerous theoretical and experimental investigations carried out in different research laboratories for many years.
For example, J. R. Bettis, R. A. House I I, and A. H. Guenter, xe2x80x9cSpot Size and Pulse Duration Dependence of Laser-lnduced Damagexe2x80x9d, Damage in Laser Materials: 1976, Glass, et al. (Editors), NBS Special Publication # 462 (1976), pp. 338-343. The scientists describe that the threshold electric field for laser-induced damage in transparent dielectrics follows an inverse relation with the fourth root of the pulse duration. Furthermore they show that the threshold follows an inverse relation with the square root of the damage spot size.
A. Vaidyanathan, T. W. Walker, and A. H. Guenther xe2x80x9cCompeting Mechanisms in Laser-Induced Damagexe2x80x9d, Damage in Laser Materials: 1979, Bennett, et al. (Editors), NBS Special Publication # 568 (1979), pp. 457-465. It reports that the experimental pulse width dependence of the damage fields at 1064 nm can be approximated in form tpxe2x88x92xc2xc.
E. W. Van Stryland, M. J. Soileau, Arthur L. Smirt, and William E. Williams, xe2x80x9cPulse-Width and Focal-Volume Dependence of Laser-Induced Breakdownxe2x80x9d, Damage in Laser Materials: 1980, Bennett, et al. (Editors), NBS Special Publication # 620 (1981), pp. 375-383. It represents the laser-induced breakdown field at 1064 nm, which was measured for various focal volumes and laser pulse width on the same samples. The results of these measurements for solid materials were fit to a simple linear dependence of the breakdown field on the product tpxe2x88x92xc2xcVxe2x88x921, where tp is the laser pulse width and V is the focal volume.
J. R. Bettis, xe2x80x9cElectron Tunnel Ionization: The Pulse Length and Wavelength Dependencexe2x80x9d, Laserxe2x80x94Induced Damage in Optical Materials: 1999, Exarhos, et al. (Editors), SPIE Volume (2000) pp. 302-316. The draftsman checking the last 30 years of the Damage Symposium reveals threshold field variation with pulse duration ranging from txe2x88x920.15 to txe2x88x920.4 with the generally accepted value of txe2x88x920.25. He does also the major conclusion that the threshold field is increased with wavelength as xcex0.77. This theoretical result isn""t entirely unexpected given the experimental evidence of numerous scientists who showed that the threshold fields increases with wavelength at about the 0.74 power.
The laser-induced damage volume (Vd) is larger than the focal volume (Vf) and it is determined by the total in-focus laser energy (E0): when E0 exceeding the threshold value is on the increase, the distinction between Vd and Vf grows. In turn the value of E0 is in direct proportion to the pulse duration, to the focal volume and to the threshold density energy Eth. Thereby for creation of small laser-induced damages it is necessary to reduce the pulse duration, the focal volume and the threshold value. However, as was shown in the articles referred to above, when the pulse duration or the focal volume grows down, the value of the threshold fields rises. Just this fact brings to using relatively large laser-induced damages for production of the images inside transparent materials.
However, the utilization of the large etch points limits the resolving capacity of laser-induced damage images. Therefore development of methods and systems for creation of small etch points and production of laser-induced damage images by using such small damages is an important, pressing problem.
U.S. patent Application Ser. No. 09/651,076 to Troitski discloses the method and the system for production of single-layer high quality portraits with comparatively small etch points. Nevertheless, these etch points have yet enough large sizes so that every etch point can reproduce different gray shades by variation of its brightness. In addition, the method bases on the fact that a portrait has single layer.
The present patent application discloses the method and the systems for production of high resolution laser-induced damage images using so small etch points that it is not possible to reproduce different gray shades by the variation of the brightness of every etch point but by the variation of the compactness of damage arrangement.
The present invention has its principal task to disclose a method, systems and apparatus for creating high-resolution laser-induced damage images inside a transparent material.
One or more embodiments of the invention comprise a method for producing a small laser-induced damage by two steps: the first step is the generation of the initial electron density, which does not exceed but is nearly equal to the critical electron density corresponding to the breakdown; the second step is generation of the breakdown at a part of the first area and control of the amount of the energy enclosed inside the plasma.
One or more embodiments of the invention comprise a method and an apparatus for producing high-resolution laser-induced damage images by two laser radiations: the first has comparatively long pulse width, its energy does not exceed damage threshold value, and it is focused at a relatively large focal spot; the second has shorter pulse duration, it is focused at a smaller focal spot and its energy exceeds appropriate threshold value.
One or more embodiments of the invention comprise a laser system for producing laser-induced damage images with small etch points by using the local control of a beam direction together with the variation of the relative position of the optical system and a work article.
One or more embodiments of the invention comprise the methods of image gradation reproduction by controlling the speed of an article movement relatively beam direction or by controlling the repetition frequency of the laser pulse radiation.